I want to make a projector show an image displayed on surfaces
In your scene, add a lamp as the type spot.

With the lamp object selected go into the texture panel, under the lamp texture, add a texture with your image. You probably want to set Extension to Extend. Set the coordinates to object and input the lamp as the mapping object.



You can also set the spotlights shape to a square by ticking Sqare in the lamps settings.



Even overlapping projector lamps work.